1) Field of the Invention
The present invention relates to an image forming apparatus that uses the principle of electrophotography for forming images, and more particularly to a fixing device in the image forming apparatus.
2) Description of the Related Art
There has been a growing requirement for saving natural resources and reducing energy consumption. For example, in image forming apparatuses that use the principle of electrophography for forming images, there is a tendency to find out a structure that enables reduction in power consumption. Particularly, the process of fixing (hereinafter, “fixing”) an image to a recording medium requires a lot of power to heat a fixing device to a considerably higher temperature. Therefore, if the fixing can be performed at lower temperature, there is a possibility to achieve reduction in power consumption.
Conventionally, the fixing is performed at a high temperature from 150 degrees to 200 degrees and it takes about one to five minutes to warm a fixing device from a room temperature. If the fixing can be performed at a temperature of 150 degrees or less, and more preferably at a temperature of around 100 degrees, it is possible to heat the fixing device with a lower power as well as warm the fixing member in a shorter time.
A softening point or a melting point of a toner has to be less than 100 degrees to achieve the fixing at a low temperature. The toner is a resin, which is an organic polymer. Generally, if the melting point of the organic polymer is low, its melt viscosity is also low. Moreover, a resin with a low molecular weight has a low melt viscosity, because, an interaction between molecules is weak. However, when the toner has a low melting point, during the fixing, the viscosity of the toner decreases abruptly as the temperature rises above the melting point of the toner so that a part of a molten toner on a recording medium adheres to a surface of a fixing roller, thereby causing an offset in the image.
One approach to prevent the offset is to use a fixing belt. The molten toner on the recording medium is made to make a contact with the fixing belt. As a result, the molten toner gets cooled and the toner coagulates so that the toner does not get adhered to the fixing belt.
However, when the fixing belt is used, to ensure stable heating of the toner and radiation of heat from the toner, it is necessary to apply pressure to the fixing belt while the toner on the recording medium is in contact with the fixing belt. If the pressure is applied is high, the fixing device requires more power to drive the fixing belt, because, a driving torque increases. On the other hand, if the pressure applied is low, a contact between the recording medium and the fixing belt becomes weak, resulting in faulty fixing such as image blurring.
Japanese Patent Application Laid Open No. H5-19646 discloses a fixing device with a cooling fan to blow a fresh air onto a surface of an endless belt (i.e., the fixing belt). A molten toner on a recording medium gets cooled faster by the fresh air and coagulates. However, a driving torque increases because of the pressure of a press-contacting roller. Moreover, it is difficult to keep a tight contact between the recording medium and the endless belt until the toner gets cooled.
Japanese Patent Application Laid Open No. H110-221982 discloses a fixing device that includes an endless fixing belt, a guiding roller, a pressure roller, a paper pressing roller, and a heating means. The guiding roller is provided at one end of a carrying path for a recording medium, and the pressure roller is provided at another end. The paper pressing roller presses the paper against the fixing belt. However, in this structure, a driving torque increases because of the pressure of these rollers, moreover, image blurring occurs because it is difficult to keep a tight contact between the recording medium and the fixing belt in a position where no rollers is present.
Japanese Patent Application Laid Open No. 2000-89593 discloses a fixing device that includes a heating roller, a fixing roller, a fixing belt extended between the fixing roller and the heating roller, and a pressure roller that presses the fixing belt from below. The fixing roller and the heating roller are respectively held in contact with the pressure roller across the fixing belt so that a nip is formed from a contact point between the fixing roller and the pressure roller to a contact point between the heating roller and the pressure roller. However, in this structure, a driving torque increases with pressure of these rollers, moreover, an image blurring occurs due to a curvature within the nip while a recording medium is being carried on the fixing belt.
Japanese Patent Application Laid Open No. H5-127551 and Japanese Patent Application Laid Open No. 2002-365948 disclose a fixing device that has a pair of belts. However, inventors of the present invention confirmed in an experiment that this fixing device produces blurred images because of undulations on a surface of the belt. Precisely, if one of the belts has the undulation on the surface, a recording medium fluctuates between both of the belts repeatedly, thereby causing the image blurring because of wrinkles on the recording medium.
Japanese Patent Application Laid Open No. 2000-19866 discloses a fixing device in which a recording medium adheres to a fixing belt because of an electrostatic force. This structure enables uniform and effective transfer of heat from the fixing belt to the recording medium.